Irradiation experience gained on fuel rods with burn-ups greater than 60 MWd/kgHM irradiated in the Nuclear Power Plant (NPP) Gosgen, Switzerland is described. Emphasis is placed on the fuel behaviour, which has been analysed by hot cell examinations at the Institute for Transuranium Elements, Karlsruhe, and the Paul-Scherrer-Institute, Villigen, in collaboration with the NPP Gosgen. Irradiation times of up to 10 annual cycles were reached giving a maximum rod burn-up of 105 MWd/kgHM. Above 60 MWd/kgHM, the so-called high burn-up structure (HBS) forms and the fission gas release increases with burn-up and fuel rod power. Examinations performed in the outer region of the fuel revealed that the HBS retains the fission gas created up to considerable porosities. Furthermore, the HBS has a relatively low swelling rate, greatly
increased plasticity, and its thermal conductivity is higher than expected from the porosity. The postirradiation investigations showed that the HBS had no detrimental effects on the performance of stationary irradiated PWR fuel irradiated to the high burn-ups reachable with 5 wt per cent U-235 enrichment. On the contrary, the HBS results in a fuel performance that is in general better than it would have been if the HBS had not formed.